1. Field of the Invention
The present invention relates to a zoom lens system suitable for use in an image-capturing optical system, such as a digital still camera and a video camera.
2. Description of the Related Art
With recent developments of sophisticated image pickup apparatuses (cameras), such as video cameras and digital still cameras with solid-state image sensors, an optical system for use in such cameras requires a zoom lens system with a large aperture ratio including a wide angle of view.
In a camera of this type, various optical members, such as a color correction filter and a low-pass filter, are arranged between the end of the lens group and an image pickup device. Therefore, the optical system for use in such a camera requires a lens system with a relatively long back focal length.
For a camera with an image pickup device for color images, an optical system with good telecentricity on the image side is desired to avoid color shading.
Conventionally, there have been proposed a variety of two-unit zoom lens systems of a so-called short zoom type. Such a two-unit zoom lens system includes a first lens unit of negative refractive power and a second lens unit of positive refractive power to perform zooming by varying the distance therebetween. In such an optical system of a short zoom type, magnification is changed by moving the second lens unit of positive refractive power, while compensation for positional variations of an image point associated with the changes in magnification is implemented by the first lens unit of negative refractive power. Such a two-unit zoom lens system has a zoom ratio of about 2.
To further achieve a compact lens system with a zoom ratio of as high as 2 or above, a so-called three-unit zoom lens system has also been proposed (see, for example, Japanese Patent Publication No. 7-3507 corresponding to U.S. Pat. No. 4,810,072 and Japanese Patent Publication No. 6-40170 corresponding to U.S. Pat. No. 4,647,160). In the three-unit zoom lens system, a third lens unit of negative or positive refractive power is arranged on the image side of a two-unit zoom lens system.
There is a known three-unit zoom lens system having a long back focal length, good telecentricity, and a wide angle of view (see, for example, Japanese Patent Laid-Open No. 63-135913 corresponding to U.S. Pat. No. 4,838,666 and Japanese Patent Laid-Open No. 7-261083).
There is also a known three-unit zoom lens system in which zooming is performed by moving a second lens unit of positive refractive power and a third lens unit of positive refractive power, while a first lens unit of negative refractive power is fixed (see, for example, Japanese Patent Laid-Open No. 3-288113 corresponding to U.S. Pat. No. 5,270,863).
There is also a known three-unit zoom lens system with a relatively small number of constituent lenses, in which all lens units are moved for zooming, and a cemented lens is effectively included in a second lens unit to correct chromatic aberrations (see, for example, Japanese Patent Laid-Open No. 2001-272602 corresponding to U.S. Pat. No. 6,498,687, Japanese Patent Laid-Open No. 2002-48975 corresponding to U.S. Patent Application Publication No. 2002008920, Japanese Patent Laid-Open No. 2003-5072, Japanese Patent Laid-Open No. 2003-149555, and Japanese Patent Laid-Open No. 2003-149556).
There is also a known three-unit zoom lens system in which a lens of negative refractive power in a first lens unit has aspheric surfaces on the object side and the image side to further reduce the number of constituent lenses (see, for example, Japanese Patent Laid-Open No. 5-323190 corresponding to U.S. Pat. No. 5,357,374, Japanese Patent Laid-Open No. 7-174971, Japanese Patent Laid-Open No. 2002-55278 corresponding to U.S. Patent Application Publication No. 2003058549, and Japanese Patent Laid-Open No. 2002-365545 corresponding to U.S. Patent Application Publication No. 2003103157).
A three-unit zoom lens system designed for 35-mm film photos is not readily applicable to an image pickup apparatus with a solid-state image sensor, because its back focal length is too long and its telecentricity is insufficient for use in an image pickup apparatus with a solid-state image sensor.
To achieve a compact camera with a zoom lens system having a high zoom ratio, a so-called barrel-retractable zoom lens system has been widely used these days. In the barrel-retractable zoom lens system, the distance between adjacent lens units is reduced during non-use to decrease the amount of projection of lenses from the camera body.
Generally, the length of each lens unit along the optical axis increases as the number of lenses in each lens unit increases. Moreover, the total lens length increases as the amount of movement of each lens unit for zooming and focusing increases. If the number of lenses in each lens unit is large, or if the amount of movement of each lens unit is large, the optical system causes an increase in the lens length when retracted, and thus cannot be used in a barrel-retractable zoom lens system.